The present invention relates to an optical pickup apparatus and an optical information recording medium recording and/or reproducing apparatus.
In recent years, there has been a rapid progress in the research and development of a high-density optical disk capable of information recording and/or reproduction, using a blue-violet semiconductor laser having a wavelength of about 400 nm. For example, an optical disk for conducting information recording and/or reproduction based on a specification of NA of 0.85 and light source wavelength of 405 nm—a so-called Blu-ray Disc (hereinafter referred to as “BD”)—is capable of recording 23 through 27 GB information per layer, as compared to the optical disk having a diameter of 12 cm, the same diameter as that of the DVD (NA: 0.6; light source wavelength: 650 nm, memory size: 7 GB). Further, an optical disk to conduct information recording and/or reproduction based on a specification of NA of 0.65 and light source wavelength of 405 nm—so-called HD DVD (hereinafter referred to as “HD”)—is capable of recording 15 through 20 GB information per layer of the optical disk having a diameter of 12 cm. When the BD is used, there is an increase in the comatic aberration caused by the skew of the optical disk. Accordingly, protective substrate is designed thinner than that of the DVD (0.1 mm as compared to 0.6 mm in the case of DVD), whereby the amount of comatic aberration caused by skew is reduced. In the present Specification, such an optical disk will be referred to as a “high-density optical disk”.
The value of the optical disk player/recorder as a commercial product cannot be said to be sufficient if information recording and/or reproduction can be conducted only for a high-density optical disk. At present, since a great variety of optical discs such as the DVD and CD (compact disks) for recording information are put on the market. Under that circumstance, adequate information recording and/or reproduction capacity must be ensured for the DVD and CD owned by the user. When this requirement has been met, it becomes possible to enhance the value of the optical disk player/recorder for high-density optical disk as a commercial product. Thus, the optical pickup apparatus mounted on the optical disk player/recorder for high-density optical disk is desired to ensure adequate information recording and/or reproduction for any of the high-density optical disk, DVD and CD.
A method has been proposed to permit adequate information recording and/or reproduction while maintaining compatibility with any of the high-density optical disk, DVD and CD. According to this method, selective switching is carried out between the optical system for high-density optical disk and the optical system for DVD and CD in response to the recording density of the optical disk for information recording and/or reproduction. However, this method requires a plurality of optical systems, and is disadvantageous from the viewpoint of downsizing and cost cutting.
To simplify the structure of the optical pickup apparatus and to reduce the cost, it is preferred in the optical pickup apparatus characterized by compatibility that standardization should be achieved between the optical system for high-density optical disk and the optical system for the DVD and CD wherever possible, thereby minimizing the number of the optical parts constituting the optical pickup apparatus. Further, standardization of the objective optical system laid opposite to the optical disk is the shortest way to simplify the structure of the optical pickup apparatus and to reduce the cost.
However, when an attempt is made to use the standardized objective optical elements to achieve compatibility in the optical pickup apparatus, different light source wavelengths are used in each optical disk. This requires some method to be devised to ensure that a light convergent spot subjected to aberration correction is formed on the optical disk information recording plane.
One of the methods for aberration correction is as follows: The coupling lens displaceable in the direction of optical axis are arranged between the light source and objective optical element. Displacement is performed in the direction of optical axis in response to the kind of the optical disk to be used. The exitance of the light flux entering the objective optical element is changed. However, in order to displace the coupling lens in the direction of optical axis, a separate actuator must be installed. To ensure the installation space, it is necessary to increase the size of the optical pickup apparatus, hence the cost increases. Such a problem remains unsolved. When the liquid crystal element is inserted between the light source and objective optical element, the similar cost problem occurs.
In another embodiment of aberration correction, a wavelength-selective diffraction structure is formed on the optical surface of the coupling lens so as to change the divergence angle or convergence of the luminous flux passing through the coupling lens in response to the kind of passing light flux, and to change the exitance of the light flux entering the objective optical element. In such a structure, the coupling lens is fixed, therefore the actuator is not necessary. However, when a diffraction structure is formed on the optical surface, the light utilization efficiency is reduced. Such a problem remains unsolved.
By contract, in the following Document 1, color aberration correction is carried out by using a doublet collimator.
[Document 1] Official Gazette of Japanese Patent Publication Application Tokkai 2005-122899
However, according to the technology disclosed in the Document 1, a doublet collimator formed by a positive lens bonded onto a negative lens is an essential constituent element. This structure increases the number of optical system manufacturing processes. In the technology disclosed in the Document 1, the optical detector is required by each light source. This leads to an increase in the size of the optical pickup apparatus and an increase in the cost.
The object of the present invention is to solve the aforementioned problems and to provide an optical pickup apparatus and an optical information recording medium recording and/or reproducing apparatus of simple structure capable of adequate aberration correction.